Method of recovering hydrocarbons from oil shale



United States Patent r1113,542,131 [72] inventors Dan K. Walton 3,1 13,620 12/1963 l-lemminger 166/257 1 Dallas: 3,139,928 7/1964 Broussard 166/261 Marlon L. Slusser, Arlington, Texas 3,233,668 2/1966 Hamilton et a1. 166/259 [21] Appl. No. 812,223 3,284,281 11/1966 Thomas 166/259 [22] Filed April 1, 1969 3,346,044 10/1967 Slusser 166/256 [45] Patented Nov. 24, 1970 3,342,257 9/1967 Jacobs et al. 166/247 [73] Assignee Mobil Oil Corporation 3,362,471 1/1968 Slusser et a1. 166/251 a corporation of New York 3,474,863 10/1969 Deans et al. 166/272X Primary Examiner-Stephen J. Novosad Attorneys-William .l. Scherback, Frederick E. Dumoulin, a al lfglti gzL s fli lg HYDRQCARBO'NS JWilliam D. Jackson, Andrew L. Gaboriault and Sidney A.

11 Claims, 1 Drawing Fig. I o [52] US. Cl. 166/257 166/259- 165/299 1661/ 303 ABSTRACT: This specification discloses a method of recover- [51 I Int. CL t l "I "L no hydrocarbons from an shale formation the in situ re. Field [66/256, torting thereof. A well penetrating the formation is heated and 247251'25635125937237], 299,302 gas is injected thereinto until a pressure buildup within the 308 well is reached due to a decrease'in the conductivity of naturally occurring fissures within the formation. Thereafter the [56] Rename cued .well is vvented in order to produce spalling of the walls thereof. UNITED STATES PATENTS This results in the formation of an enlarged cavity containing 2,630,307 3/1953 Martin 166/251 rubblized oil shale. A hot gas then is passed through the rubbl- 2,780,449 2/1957 Fisher et al. 166/259 ized oil shale in order to retort hydrocarbons therefrom and 3,001,775 9/ 1961 Allred these hydrocarbons are recovered from the-well.

raw-un- P tented.- Nov. '24, 1970 may:

ATTORNEY V 1' METHOD F'RECOVERINGHYDROCARBONSFROM OILSI-IALE J BACKGROUND OF THE INVENTION This invention relates to the recovery of hydrocarbons from oil shale deposits and more particularly to the recovery of hydrocarbons by the in situ retorting of such deposits.

Oil shale deposits are shale formations which contain hydrocarbons in the form of kerogen. Kerogen is a solid or semisolid substance formed of a complex mixture of organic compounds. While lterogen is for all practical purposes substantially immobile within the shale formation, it is well known that liquid and gaseous hydrocarbons can be derived from kerogen upon the application of heat. Accordingly, most in situ extraction techniques involve the introduction'of a heating medium into the oil shale deposit in order to retort the kerogen and .thus conv'e'rti't into mobile hydrocarbons which can be withdrawn to the surface of the earth through suitable wells. Heating typically is accomplished either by'the injection 'of air in an in situ combustion process wherein a portion-of the kerogen is burned tosup'ply the desired heat or by the injection of hot, inert gases, heated either atthe surface or within the injection well. In any case, the oil shale is heated to a temperature sufficient'to retort hydrocarbonstherefrom by distillatio n and/or'decomposition of the kerogen.

Numerous thermal techniques have been proposed for the in situ extraction of hydrocarbons from oil shale. For example, in US Pat. No. 3,139,928 to Broussard there is disclosed a recovery process in which an oxygen containing gas such as air shale is disclosed in U.S: Pat. No. 3,346,044 toSlusser. In this technique,'a heating gas is injected through a well andinto the oil shale formation until such time as a peripheral pressure is injected through a well and into an oil shale deposit to effect tion immediately adjacentthe well in order to develop a frail ash skeleton of rock about the initial cavity. An explosive is again detonated within the well in order to knock down this skeleton of rock and the process thereafter may be repeated as desired. In another in'situ extraction technique disclosed in U.S.- Pat. No. 2,630,307 to Martin, spaced injection and production wells are drilled into the oil shale formation. Explosives are detonated at the bottom of these wells in order to form cavities and to provide a fractured and cracked permeable zone between the wells to enhanc'ethe flow of fluids through the formation. Thereafter, a combustion zone is established adjacent the injection well and hydrocarbons derived from the retorting of the oil shale are withdrawn through the production well.

Numerous techniques in addition to those described above have been proposed for the recovery of hydrocarbons from oil shale deposits. For further examples of such techniques, reference is made to US. Pat. Nos. 3,302,707 and 3,362,471 to Slusser et a1.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a new and improved process for the in situ extraction of hydrocarbons from an oil shale formation wherein a'cavity is formed containing rubblized shale, which shale then is subjected to retorting in order to recover hydrocarbons therefrom. The present invention is carried out in an oil shale formation penetrated by a well. The well is heated and gas is injected into the well in order to establish an elevated pressure therein. After the desired pressure is reached, the well is vented in order to reduce the pressure therein at a rate sufficient to produce spalling of the walls of the well. This produces an enlarged cavity which contains rubblized shale resulting from the spalling of the walls. Thereafter, a gas is injected through a passage within the well and passed through the rubblized shale at a temperature sufficient to retort hydrocarbons therefrom. Concomitantly with the injection of the gas, the hydrocarbons derived by retorting of the rubblized oil shale are withdrawn from the well through another passage therein.

seal is formed at a distance from the well. This seal, which is due to the application of heat to the oil shale, functions to close off naturally occurring fissures within the deposit. The

well then is vented in order to recover hydrocarbons fromthe well. This process may be repeated at increasing pressures in order to advance the pressure seal outwardly into the formation and alternately recover'hydrocarbons. Ultimately gas may be injected into the well under pressures sufficient to fracture the oil shale deposit and cause disruption of the peripheral pressure seal. The process then may be extended to further portions of the deposit beyond the disrupted pressure seal.

'Somein situ extractiontechniques involve the formation of cavities within the oil shale deposit'either deliberately or as an incidental result'of the process. For example, the technique disclosed in the aforementioned patent'to Broussard is said to involvethe formation of a permeable cavern"near the bottom of the well. While the mechanism by which'this cavern is formed is not disclosed, it apparently is formed early in the process, possibly during the initial heating step. Other techniques in which caverns are deliberately produced in subterranean oil shale deposits are disclosed in U.S; Pat. No. 3,001,775 to Allred and. US. Pat. No. 3,113,620 to Hemminger. In the Allred process, a cavern in formed at the bottom of the deposit by fracturing and dissolving the shale with an acid. Explosives then are detonated in shot holes immediately above the cavern in order to cause the shale to fracture and collapse into the cavern. Thereafter, one or more holes are drilled into the base of the fractured zone and an in situ combustion process is carried out. In the l-lernminger process a small cavity is formed in an oil shale formation by first detonating an explosive charge at the bottom of a well. An in situ combustion process then is carried out in'the forma- In a preferred embodiment of the invention, gas then is injected through a passage within said well at a higher injection rate than that employed during the retorting step in order to circulate solid debris remaining after the retorting step from the well. Thereafter, gas is again injected into the well in order to establish an elevated pressure within the cavity and the process is repeated.

In a further aspect of the invention, the above described procedure is carried out in two or more wells penetrating the same oil shale formation and repeated in each of the wells until cavities of the desired dimensions are produced. Thereafter the oil shale formation is fractured in order to establish fluid communication between the wells. Combustion then is established within the formation and a combustion supporting gas is injected through one of the wells while the other well is utilized as a production well for the recovery of hydrocarbons.

BRIEF DESCRIPTION OF'THE DRAWING The drawing is a vertical section of an oil shale formation 1 showing a plurality of wells penetrating the formation.

DESCRIPTION OF SPECIFIC EMBODIMENT which is formed at the bottom thereof during a latter stage of this invention.

The well is provided with a casing 18 which is cemented within the well as indicated by cement sheath 19. The casing 18 and its surrounding cement sheath preferably extend into formation 16 as shown in order to provide a good seal and to prevent fluids from traveling upwardly behind the casing where they may be lost to the surface or into the overburden 20. However, in some cases the casing may safely be landed above the top of the oil shale formation and in rare instances the casing may be eliminated entirely. Usually, however, it will be desirable to provide some casing within the well in order to prevent collapse of the overburden into the well. The well 10 also is provided with a tubing string 22 which extends from the wellhead 24 to a lower elevation within the oil shale formation. Preferably, the tubing 22 is landed adjacent the bottom of the well as shown for reasons which will appear hereinafter. The tubing 22 defines one passage for the flow of fluids through the well, and the well annulus 25 externally of the tubing provides another such passage. At the wellhead, a flowline 26 extends from the tubing 22 to provide for the introduction and/or withdrawal of fluids. The casing 18 is provided with a flowline 27 which serves a similar purpose. The well 12 is identical in its structural components to well 10 and like elements in well 12 are designated by the same reference numerals as used with respect to well 10, subscripted by a.

In carrying out the first step of the invention, the open zone of the well below the casing 18 is heated and gas is injected into the well through line 26 and/or line 27 in order to establish an elevated pressure therein. Preferably, an oxygen containing gas such as air is injected into the well under conditions leading to combustion of kerogen in formation 16. Combustion may be initiated by any suitable technique. For example, a gas-fired or electric heater may be supported on the lower end of tubing 22 in order to heat the adjacent portion of formation 16. Simultaneously with or subsequent to this application of heat, the combustion supporting gas is injected into the well. In some oil shale formations combustion may be established by autooxidation. In this technique, air, usually enriched with oxygen, is injected in order to slowly bring the kerogen up to the combustion temperature without the use of extraneous heating means.

While it is preferred that the first step of the invention be carried out as an in situ combustion operation, the heating and prcssurizing ofthe well can be accomplished by other suitable techniques. For example, an inert gas such as flue gas or natural gas may be injected into the well. The gas may be heated at the surface and/or heat may be supplied by means of a heater located within the well.

As heat is applied to the formation adjacent the well 10, the oil shale becomes progressively less permeable. In all likelihood, one factor leading to this decrease in permeability is thermal expansion of the oil shale which leads to narrowing and possibly even closing of naturally occurring fissures or fractures therein. However, it would appear that other mechanisms also are involved since the reduction in permeability is partially irreversible, i.e., when oil shale is heated to an elevated temperature and thereafter cooled to its original temperature, the permeability does not return to its original state. It seems likely therefore that the reduction in permeability occasioned by heating of the oil shale is due in part to a change in the condition of the kerogen. It is believed that the kerogen or thermal derivatives thereof form deposits within the fissures in the oil shale which function in conjunction with the thermal expansion to restrict flow through these passages.

The degree of permeability reduction in the fracture network of the oil shale depends upon the temperature to which the oil shale is heated and the duration of heating. Experiments carried out regarding the invention indicate that any appreciable elevation in temperature will result in some decrease in conductivity of the fractures. However, a significant reduction in conductivity within a relatively short time period does not begin to occur until the temperature reaches a value of about 300F. Therefore, it is preferred in carrying out the present invention to heat the oil shale to a temperature of at least 300F. in conjunction with the injection of gas into the well.

As the conductivity of the fracture network in the oil shale is decreased, the injection of gas results in a pressure buildup within the well. Gas injection is continued until the pressure within the well adjacent the formation reaches the desired elevated pressure. Thereafter the second step of the invention is carried out. In this step, the well is vented to the surface through line 26 and/or line 27 as appropriate. For example, if the gas has been injected through the tubing string 22, the gas injection step is terminated and line 27 is open to the atmosphere or to a low pressure collection zone such as a large tank (not shown). By rapidly reducing the pressure within the well, a spalling action is produced which causes the walls of the well to crumble to produce an enlargement or cavity within the formation 16. The resulting oil shale particles which fall to the bottom of the cavity form a permeable body of rubblized shale which is susceptible to further recovery procedures for the extraction of hydrocarbons therefrom. The degree of spalling depends upon the pressure reached during the initial gas injection step. Accordingly, it is desirable to raise the pressure during this initial step to as high a level as possible consistent with the available injection equipment, economy of operation, and the fracturing characteristics of the particular oil shale formation involved. The pressure should not reach that at which fracturing of the oil shale formation occurs. Usually it will be possible to economically reach a value of at least 600 psi. with the injection equipment available and without fracturing of the oil shale formation, and it is preferred to elevate the pressure to at least this level during the initial gas injection step.

After increasing and then decreasing the pressure in the well one or more times to produce a cavity of the desired size, the third step of the invention is carried out whereby the rubblized oil shale within the cavity is subjected to in situ retorting. This phase of the invention will be described with reference to well 12 in the drawing. The cavity formed by the first two steps is indicated by reference numeral 30. As illustrated, the cavity 30 contains a body of rubblized oil shale 32.

The tubing string'22a by virtue of its having been landed near the bottom of the well provides a passage into the interior of the rubblized oil shale 32. In this step of the invention, a retorting gas is injected through one of the passages defined by tubing 22a and annulus 25a and passed through the rubblized oil shale 32 at a temperature sufficient to retort hydrocarbons from the oil shale. The hydrocarbons thus derived are withdrawn from the well through the other passage. By this arrangement, the injected retorting gas and the fluid hydrocarbons produced are moved vertically through the rubblized oil shale 32 and an appreciable quantity of the rubblized oil shale is contacted by the retorting gas.

As a general rule, the hydrocarbons produced by retorting are primarily gases and liquids entrained in gas and it will be most expeditious to withdraw these hydrocarbons through the well annulus 25a. Thus, it usually will be preferred to inject the retorting gas through tubing 22a. However, the reverse mode of operation may be utilized, particularly where appreciable amounts of the retorted hydrocarbons are in the liquid state at the conditions existing within the well. In this case the bottom of the cavity 30 will serve as a sump from which liquid hydrocarbons can be withdrawn through tubing 22a.

The retorting step should be carried out at a temperature on the order of 750F. or more in order to effect derivation of an appreciable amount of hydrocarbons from the kerogen by distillation and/or decomposition thereof. These temperatures can be readily achieved by passing a hot, inert gas through the rubblized oil shale or by the injection of a combustion supporting gas and either mode of operation may be used in carrying out the invention. It is preferred, however, to effect retorting by in situ combustion for reasons of economy and since this results in a mass of fine particulate material which is relatively free of carbonaceous residue remaining after the retorting step.This material can be easilyrernoved from the well as described in greater detail hereinafter.-

The combustion supporting gas is injected into the'rubblized oil shale 32 through tubing 22a and combustion is in itiated. ln mostinstances combustion 'ca'n' be established by auto-oxidation due'to the already heated condition of the rubblized oil shale 32; However, if necessary ;-'a heaterrnay'be employed to heat the oil shale to a temperature sufficient toi'nitiate combustion. As combustion of the kerogen takes place, the fluid hydrocarbons derived from distillation" and/or decomposition of the kerogen and the products of combustion are withdrawn from the well through annulus a and passed through flowline 27a to a suitable collection zone.

After completion of the retorting step, it usually 'well be desirable to repeat the process one or more times. in this case, it is preferred that thesolid debris remaining after fretortingbe "removed fromthe cavity '30 inorder to prepare it forfurther enlargement and the accumulation of additional rubblized oil shale. This may 'beaccomplished by'injecting gas through the tubing 22:: at a rate sufficient to circulate'the-solid debris remaining after the'retorting step from the wellQFor example,

as combustion is ended, the air injection rate'can be'increased to the desired level and maintained at this level until an'examination of theeffluentfrom line 27E: showsthat the debris has been removed from cavity 30. Thereafter,the'pressure within the well is increased to institute a new cycle of opera,-

; tron.

In those cases in which the inventionis applied ina plurality of wellspenetrating an oil shale formatiomthe cavitiesformed can be utilized to advantage in flowing fluids to'and from the formation. Thus, in'a further aspect of the invention,'the oil shale formation is fractured intermediate wellsin which the above described process has'been' carried out with the resultant formation of cavities. Thereafter, andins'itu combustion operation is carried out inthe'fracturedportion of 'the deposit utilizing one or more of the wells for'injec'tion of the combustion supporting gas and the remaining wells for production of hydrocarbons.

More particularly and with'furthe'rreference to thedrawing,

the single: well reto 't pr'o'cess iscarri'ed out to' c'onipletion' in' d. concomitantly with step c withdrawing said hydrocarbons from said well through another passage therein.

2. The method of claim 1 further comprising repeating steps a, b, c, and d. l

3. The method of claim 1 further comprising subsequent to step d injecting gas through a passage within said well at an increased" injection rate sufficient to circulate solid debris remaining after said retorting step from said well and thereafter repeating steps a, b, c. and d.

4. The method of claim 1 wherein said well is heated in step a toe temperature of at least 300F. and the elevated pressure established in step a is at least 600 p.s.i.

5. The method of claim 1 wherein the gas injected in step a is a'combus'tion supporting gas and the well is heated by combustion in said oil shale.

- through said rubblized shale.

'8. The method of claim 7 wherein saidgas is injected through the passage'defined by said tubing whereby the gas wells 10 and 12 resulting in th'ecavity 30 at meii'onom-or well l2 and a similar cavity (not shown) at the bottom of well 10.

In addition, an explosive is detonated in one or more shot holes located intermediate the wells '10"and 12. Thus; as

shown in the drawing,-a shot hole'36hasbeen'drilled' into for mation 16. Upon detonation of an explosive at the'bottomof hole 36,the formation isfractured and a body of rubblized shale is formedbetw'eenthe cavities at the lower ends of wells 10 and 12'. A combustion front is thenini tiated adjacent one of the wells, anda combustion supporting gas' is injectedthrough well. 10. Hydrocarbonjsretorted.fromthe oil shale and the products of combustion are recovered throughwelln which is operated as a production well. The "combustion process maybe operated as either a direct or" indirect drive.

Thus, in a direct drive, combustion is initiated adjacent the'injection well 10 and advanced through the formationin'a direction of the production-well l2. lf-the process'iiscarried out as an indirect drive, cornbu'stionis initiated adjacent the production well l2"an'clv then advanced through the zone toward the injection well 10 by the injection" of air therethrough.

We claim: I 1. In the recovery of hydrocarbons'from anoil shale formation penetrated by a well,.'the method comprising:.

a. heating said well and injecting gas thereinto until'an elevated pressure is established within said well; b. thereafter venting said well to reduce the pressure within said well at a rate sufticient toproduce' spalling of the walls thereofwhereby and enlarged cavity is formed containing rubblizedshale resulting from saidspalling;

c. thereafter injectinga gas through passage within said well and flowing-said gas through said rubblized shale ata temperature sufficient to retort hy'drocarbons therefrom and tion'penetrated by a well, the method comprising:

' a. injecting a combustion supporting gas into said well and effecting combustion in said oil shale until a temperature of at least 300F. and apressure of at least 600 p.s.i. are established within said well;

b. thereafter reducing the pressure within said well at a rate sufficient to produce 'spalling of the walls thereof whereby an enlarged cavity isformed containing rubblized shale resulting from said spalling;

i c. thereafter injecting a c'ombustion'supporting gas through -a passage within said well and through said rubblized shale and effecting combustion in said rubblized shale to retort hydrocarbons therefrom;

. concomitantly with step c withdrawing said hydrocarbons from said well through another passage;

e. thereafter injecting gas through a passage within said well at an increased injection rate sufficient to circulate solid debris remaining after retorting of said rubblized oil shale from said well; and

f. repeating steps a, b, c, and d.

' 10! In the recovery of hydrocarbons from an oil shale formation'penetrated by at least two wells, the method comprisa. heating each of said wells and injecting'ga's thereinto until an elevated pressure is established .within each of said wells;

' b. thereafter reducing the pressure within each of said wells at a rate suff cient to produce spalling of the walls thereof whereby an enlarged: cavity is formed containing rubblized shale resulting from said spalling;

c. thereafter injecting a gas through a passage within each of said wells and through said rubblized shale at a temperature sufficient to retort hydrocarbonstherefrom and concomitantly withdrawing a mixture of gas and retorting hydrocarbons from each of said wells through another passage therein;

. thereafter injecting gas through a passage within each of saidwells at an increased injection rate sufficient to circulate solid debris remaining after said retorting step from each of said wells; v

e; thereafter repeating steps'a, b, and c at leastonce in each of said wells; 1

f. fracturing said oil shale formation intermediate said wells;

and

11. The method of claim 10 wherein said formation is fractured by detonating an explosive within located intermediate said wells. 1

at least one shot hole 

